Infra-Through Ultrasonic Piezoelectric Acoustic Vector Sensor

Hindawi Publishing CorporationSmart Materials ResearchVolume 2012, Article ID 356190, 6 pagesdoi:10.1155/2012/356190

Research Article

Infra-Through Ultrasonic Piezoelectric Acoustic Vector SensorParticle Rejection System

Scott E. Cravens and Ronald M. Barrett

Aerospace Engineering Department, University of Kansas, Lawrence, KS 66045, USA

Correspondence should be addressed to Ronald M. Barrett, [email protected]

Received 15 November 2011; Accepted 16 January 2012

Academic Editor: Tao Li

Copyright © 2012 S. E. Cravens and R. M. Barrett. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Sensor elements which employ fine filaments are often vulnerable to particulate fouling when used in certain operational fieldconditions. Depending on the size, attraction level, thermal and electrical conduction, and charge accumulation properties of theparticles, erroneous readings can be easily generated in such “dirty” environments. This paper describes the design, development,and testing of an ultrasonic system which dynamically rejects highly tenacious electrostatically charged particles of a wide varietyof sizes and even water. The paper starts with a brief introduction to the field of acoustic vector sensing, outlining its outstandingcharacteristics and history. Operational challenges including a statistical analysis of typical Middle-Eastern wind-blown desert sandand charge density are laid out. Several representative subscale hot-wire filaments were fouled with calibrated dust representingdesert sand. The fouled elements were then exposed to airflows of 13 ft/s (4 m/s) and showed highly erratic shifted conductionlevels with respect to baseline (clean) levels. An ultrasonic cleaning system was designed specifically resonate the filament andcantilever so as to mechanically reject foulants. When operated at resonance, the ultrasonic cleaning system showed 98.6%particulate rejection levels and associated restoration of uncorrupted filament resistance levels to within 2% of baseline resistancemeasurements.

1. Introduction

Over the past decade, a new class of acoustic sensorshas evolved. Conventional microphone technologies simplymeasure pressure as a function of time. These scalar mea-suring devices must be used in sizable clusters with powerfulcomputers detangling this limited information [1]. A newapproach using acoustic vector sensors (AVSs) employsfundamentally different sensor physics. These AVS elementsare capable of not only determining pressure as a function oftime (as do conventional microphones), but they can divideacoustic source directionality by measuring particle velocityin x, y, and z coordinates with time at very high rates andphenomenal dynamic range [2]. Acoustic vector sensors use anumber of different techniques to determine acoustic sourcedirection. One of the leading acoustic vector sensors employsan advanced dual hot-wire anemometry setup to measureparticle movement in multiple directions. The device is usedin many applications ranging from acoustic holography to

tracking airborne vehicles as well as aquatic applications.Most of the applications currently employing these AVSdevices are limited to clean environments, meaning free ofdust particles, and other small environmental debris. Testingin dirty environments is limited at best to date. Fouling byparticulate adherence or even complete particle blockage ofmeasurement channels is a very distinct possibility given thesmall scale of the devices. The primary vulnerability comesfrom the very geometry and size of the sensor filamentswhich when compared to the size of airborne particulatesare such that the largest airborne particles are an exactfit between the substrate and the sensor elements, whichindicates that the devices could actually “trap” airborneparticles. Figure 1 shows an interesting comparison of typicalAppalachian quartz-based beach sand and sand from the AshShuqqan in the Empty Quarter (Rub al Khali) of the ArabianPeninsula.

The reader will note a profound difference between thetwo types of sand as the beach sand is quite rounded and

2 Smart Materials Research

Destin, FL beach sand Windblown Rub al Khali sand

Figure 1: Beach sand from Destin, Florida (for reference) and windblown sand from the Rub al Khali, Arabian Peninsula.

908070605040302010

0

Destin, FLRub al Khali

1%8% 8%

3%12%

5%15%

23%

83%

36%

0%6%

0.01–0.1 0.1–1 1–10 10–100 100–1000 1000–

Part

icle

s by

mas

s (%

)

Particle size, dp (micron)

Figure 2: Particulate sand size distribution by percentage of samplemass of beach sand and Middle-Eastern windblown sand.

80th percentile sizes of windblown sand

Acoustic vectorsensor filaments

(to scale)

Figure 3: Advanced AVS sensing wires [2].

fairly even in size distribution with more than 80% of thegrains falling between 130 and 640 μm in diameter. The windblown sand, on the other hand, is quite different with awide distribution of sizes ranging widely from less than 1 μmto 1100 μm in diameter in statistically significant numbersby weight and volume. Another interesting characteristic isalso seen in Figure 1: Static electric attraction. Wind blownsand from the Arabian Peninsula on the other hand behavesvery differently and possesses different properties. Becausemost beach sand has been completely coated by salt waterfor a very long time, it is more resistant to holding largestatic charges. Indeed, single grain electrical resistance levelsin 30% humidity were measured at 300–500Ω/mm for thebeach sand and more than 20 MΩ/mm for the wind blownsand. Similarly, the capacitance levels (i.e., ability to hold acharge) were 7 orders of magnitude greater for wind-blownsand. Figure 1 shows a comparatively tight dome distributionwith mostly only larger grains falling past the edges of thepile. This is because electrostatic attraction generally holdsthe pile, especially the smaller particles, together. Figure 2shows a statistical distribution of sizes from the two samples.

Boronfilament

PZT

+/−

−/+

Substrate

0.4 (10.2 mm)

0.7 (

17.8

mm

)

1.05

(26.

7 m

m)

Figure 4: Piezoelectric cleaning device dimensions (in inches).

Boronfilament

PZT cleaning element

Figure 5: Assembled cleaning device.

The biggest challenge posed by windblown sand can beseen when compared to the physical size of an advancedAVS sensor element. Considering 1 mm long platinum sensorelements with a 5 μm × 200 nm cross-section, it is easy tosee that the wide distribution of sizes of windblown sandpresents not only a great threat of electrostatic cling, butphysical entrapment between the filaments and the substrateitself. From Figure 3, it is easy to see that as air flows fromlower left to upper right and back if it were bearing particleswith size distributions laid out in Figure 2, serious challengescould be posed to the sensors. Figure 3 shows that the size ofthe desert sand goes well into the “dust” range with particlesranging down to fractions of a filament diameter, leading tocling of charged particles to filaments.

Because such atmospheric particles can render thisremarkable sensor ineffective, this study is centered onmaking a system which can skirt these problems by effectivelyand safely cleaning the filaments of all foulants. References[3–5] highlight the point that there is relatively little in theopen literature on infra-through-ultrasonic particle rejectionknown in the open literature.

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Boron filament

Figure 6: Boron fiber before and after cleaning with 20 kHz ultrasonic vibrations.

Boronfilament

Figure 7: Boron fiber before and after cleaning—calibrated 5–30 μm diameter particles.

2. Proof-of-Concept Testing on Boron Filaments

The chosen method of cleaning the AVS sensor is to usepiezoelectric elements to induce infrasonic, sonic, and ultra-sonic vibrations to mechanically knock off particulate con-taminants. As a proof-of-concept exercise, Lead-ZirconateTitanate (PZT) was designed into a “micro-shaker.”

To test the concept in a variety of environments a range offilament diameters were chosen as well as a range of particlesizes. The first filament tested represented a large-scale ananalog to the AVS sensor. A 127 μm diameter boron fiberwas fixed to a brass substrate and PZT extensional actuatorelement. The basic dimensions (in inches) of the deviceconstructed are shown in Figure 4 and the finished deviceis shown in Figure 5. Teflon tape was used to allow for lowfriction longitudinal motion.

To observe the fiber, the assembled device was placedunder a microscope. Excitation of the PZT strip was achievedusing a signal generator and a linear voltage amplifier. Figureshows a fiber which was coated with 5–30 μm diameterdust particles under the microscope before and after thePZT strip was excited. A 20 kHz sine wave with a peak topeak voltage of 20 V was used to excite the PZT, generatinglocal filament accelerations in excess of 350 g’s. Although nodeflections were visible during the cleaning event, the effectwas pronounced.

Although not clearly visible in Figure 6, the dust usedfor the first test was microscopically fibrous in nature.Still, it possessed conductivity, capacitance levels and chargeaccretion properties which were within 20% of the Rub alKhali desert sand sample. Figure 7 shows the Boron fiberwhich had been dusted with nonfibrous silicate particles

before and after cleaning. As with the fibrous foulants,the conductivity, capacitance levels and charge accretionproperties which were within 20% of the Rub al Khali desertsand sample.

3. Anemometer Wire Fouling and Cleaning

It has been shown that debris can be successfully removedfrom a small diameter Boron filament using PZT to vibratethe wire. The following sections extend this result to ahot wire anemometer, which is more representative of AVSelements. Two different anemometers were constructed, oneusing a copper sensing wire and another using a nickeltitanium alloy wire.

3.1. Nickel Titanium Alloy Filament Anemometer. Nickeltitanium alloy (NiTinol) was used for the second typeof anemometer constructed. This selection was chosendue to its ideally small diameter of only 38 μm. For thisanemometer, copper-conducting posts were soldered to acircuit board and the NiTiNOL was fixed to the ends of theposts. Figure 8 shows the basic configuration of the NiTiNOLanemometer. The guide posts were later discarded when itwas determined that they would not affect the results.

To vibrate the NiTiNOL anemometer, a small stand wasbuilt that supported a PZT bending element. The bendingelement consisted of PZT strips bonded to either side of analuminum substrate. This configuration resulted in verticaldeflections of the anemometer when the actuator is parallelto the ground. Figures 9 and 10 show the NiTiNOL filamentmounted on the PZT bending element.


(7.6 mm)

(12.7 mm)

SMA wire

0.5

1.3 (33 mm)

1 (25.4 mm)

0.3

0.2 (5.1 mm)

Figure 8: Nickel titanium alloy anemometer.

3.5 (8.9 cm)

Figure 9: NiTiNOL anemometer mounted on PZT bendingactuator.

0.16 (4

mm)

Figure 10: NiTiNOL anemometer.

3.2. Experimental Setup. A small blowdown type wind tun-nel of 2′′ inner diameter was built to test the properties ofthe AVS cleaning system as shown in Figure 11. This setupprovided a consistent airflow at a velocity of up to 4 m/s.Data was collected using a laptop computer with LABViewdata acquisition software. This was only used to record thetime history of the resistance or current running through theanemometer.

A 1F capacitor arranged in parallel with the power supplyprovided clean power. For the NiTiNOL tests, a constantvoltage was applied across the circuit. Testing was conductedfrom 50 to 250 mA (with filament failure being experiencedat 270 mA. Current was measured by tracking voltage dropacross a 1Ω power resistor as shown in Figure 12.

SMAfilament Variable speed fan

Flow

2 (5.1 cm) φ air duct

Figure 11: Wind tunnel setup.

Regulatedpower supply

+

−1F

ΔV = I

Anemometer

1Ω

Figure 12: NiTiNOL anemometer circuit.

3.3. Baseline NiTiNOL Anemometer Testing and Results. Asa baseline for the performance of the anemometer, the fanwas cycled on and off several times without any cleaningexcitation. The actuator was then excited at 200 Hz and thefan was cycled on and off again. Figure 13 shows the currentflow through the filament during the fan cycles for the staticand vibrating case. The peak voltage for the vibrating casedrops a small amount during the cycles, but the amplitudeof voltage change is approximately the same over each cycle.This indicates that the vibrations have little effect on theperformance of the anemometer.

Figure 13 shows the overall peak-to-peak performanceof the actuator by two phase shifted signals. The reader isasked to note that the measurements that are most importantfrom Figure 13 are the peak-to-peak values and intermediatesignal ripple, not the degree of phase shift.

After testing the performance of the clean anemometer,calibrated 5–30 μm diameter nonfibrous dust was thenapplied to the wire and the performance was checkedagain. Figures 14, 15, and 16 show the resulting change in

Smart Materials Research 5

0.25

0.24

0.23

0.21

0.2

0.22

0.19

0.18

0.17

0.16

0.150 10 20 30 40 50 60 70

Static

Time (s)

Cu

rren

t fl

ow,I

(A)

(a)

0.25

0.24

0.23

0.21

0.2

0.22

0.19

0.18

0.17

0.16

0.15

Vibrating

0 10 20 30 40 50 60 70

Time (s)

Cu

rren

t fl

ow,I

(A)

(b)

Figure 13: Baseline clean NiTiNOL anemometer before dusting.

0 Hz 3.5 ft/s (1.07 m/s)

200 Hz3.5 ft/s (1.07 m/s)

Figure 14: NiTiNOL anemometer filament dusted with 5–30 μm particles and under 200 Hz excitation (for photographic reference).

3 mil (76 micron) φ SMA filament

Figure 15: Anemometer after cleaning with infra-through ultra-sonic vibrations.

performance for the anemometer. Each test was conducted inthe following manner and order. Dusted, vibrating, and aftercleaning refer to the curves in Figures 14, 15, and 16.

(1) Dusted: the calibrated 5–30 μm diameter dust wasapplied and the fan was cycled on and off with outany vibration.

0.25

0.23

0.21

0.19

0.17

0.150 10 20 30 40 50 60 70 80 90 100

Dusted

(N.B. 3 separate test runs-no phase correlation)

Dusted

VibratingAfter cleaning

Baseline

Cleaned

Time (s)

Cu

rren

t fl

ow,I

(A)

Figure 16: Baseline, dirty, and cleaned current flow throughanemometer filament.

(2) Vibrating: the actuator was excited (200 Hz at 7 Volts)and the fan was cycled on and off.

(3) After cleaning: the actuator was turned off and the fanwas cycled on and off.


Figures 14, 15, and 16 show the quiescent 38 μm diameterNiTiNOL filament suspended between two support posts,covered with calibrated 5–30 μm diameter charged particles.Live testing of the cleaning system was conducted in a4 m/s airflow. Infrasonic through ultrasonic vibrations of 10–20 kHz clearly rejected electrostatically attached particles asshown in Figures 14, 15 and 16.

Clearly from Figure 16, it has been shown that fouledanemometer filaments generate highly erratic unreliablereadings with current flow shifts on the order of 35 mA.Given an anemometry sensitivity level on the order of400 m/s/A, this represents a signal corruption level of14 m/s. Infrasonic through ultrasonic cleaning rejected suchparticulate fouling and resulting signal corruption, leadingto restoration of performance levels to within 2% of baselineperformance.

4. Conclusions

It has been shown that some of the most advancedmicroanemometer and acoustic vector sensor (AVS) ele-ments are vulnerable to signal corruption when exposedto typical windblown particles. A statistical analysis ofwindblown desert sand from the Arabian Peninsula showedthat statistically significant percentages of particles rangedfrom just 1/100th of the diameter of a typical AVS sensorfilament to large enough to become lodged between thefilaments and even between the filaments and mountingsubstrate. Wind tunnel tests on 38 μm diameter filamentsshowed anemometry sensitivities on the order of 400 m/s/Awere achieved. These sensitivities were dramatically cor-rupted when the filament was exposed to 5–30 μm diameterparticles which were shown to electrostatically cling to thefilament, junctions, and post. In addition to erratic signalsdue to dusting an erroneous shift resulting in a 14 m/smeasurement error appeared due to particulate fouling. Aninfra-through ultrasonic cleaning system was designed toreject these particles. When energized and swept througha broad frequency range, foulants were expelled from thesensor filaments, restoring sensor performance to within 2%of baseline levels.

Acknowledgment

The authors would like to thank the University of KansasTransportation Research Institute for the continued supportand funding of this and related research.

References

[1] D. R. Fulton, P. A. Hawes, and K. S. Lally, “Multiple AcousticThreat Assessment System,” International Patent ApplcationNo. PCT/US2009/039909.

[2] H. E. De Bree, “A perspective on acoustic vector sensors inpassive surveillance—real world measurements, algorithms andapplications,” in Proceedings of the Aero India, Bangalore, India,2009.

[3] D. Ensminger, Ultrasonics: Data, Equations, and Their PracticalUses, vol. 10, CRC Press, Boca Raton, Fla, USA, 2009.

[4] D. Williams, Guide to Cleaner Technoogies: Cleaning andDegreasing Process Changes, US Environmental ProtectionAgency (EPA), Washington, DC, USA, 1994.

[5] G. K. Nikas, “A mechanistic model of spherical particleentrapment in elliptical contacts,” Proceedings of the Institutionof Mechanical Engineers, Part J, vol. 220, no. 6, pp. 507–522,2006.

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